Photoinduced charge shift as the driving force for the excited-state relaxation of analogues of the Photoactive Yellow Protein chromophore in solution

Transient absorption spectroscopy with subpicosecond laser excitation is used to probe the primary photoinduced processes in two ester analogues (linear and cyclic) of the Photoactive Yellow Protein (PYP) chromophore in solution. The PYP chromophore is the thioester derivative of the deprotonated trans-4-hydroxycinnamic acid. The results found for the ester analogues are compared to those previously obtained for the deprotonated trans-4-hydroxycinnamic acid and its amide and thioester derivatives. Special attention is paid to the role of the electron donor-acceptor character of the chromophore substituents and of the molecular flexibility on the excited-state relaxation pathway and kinetics. Solvent viscosity and polarity effects on the kinetics are also analyzed. Two hypothetical relaxation pathways involving a one-bond flip mechanism are proposed to explain the observation of a transient species in the course of the excited-state relaxation of the analogues bearing the stronger electron-acceptor substituents. In the first one, the intermediate is described as a perp ground state, whereas the second one involves a twisted excited state where the conformation of the ethylenic bond deviates from 90°. In both cases, the relaxation of the transient state may lead or not to the cis isomer.